1. Field of the Invention
The present invention relates to a method and an apparatus for removing foreign matters, which have adhered to a shadow mask during manufacturing of a cathode-ray tube such as a color cathode-ray tube, using an electron gun of the cathode-ray tube itself.
2. Description of the Prior Art
When a foreign matter adheres to a shadow mask, an area corresponding to the foreign matter on a screen appears as a black dot in a raster or areas with other colors radiate. Great care is, therefore, taken in the process of manufacturing so as not to allow foreign matters to adhere.
When a color cathode-ray tube, for example, is concerned, after a shadow mask is attached to a face panel, the face panel is fused with a funnel using a low-fusion point glass in order to achieve evacuation lock.
Since an effective method for removing foreign matters, which have adhered to the shadow mask, after an evacuation lock process has not been devised, defectives sometimes occur.
As a countermeasure against the foregoing drawback, a method for removing foreign matters by applying mechanical vibrations has been adopted as a method for removing foreign matters, which have adhered to a shadow mask, after the evacuation lock process. Similar methods are disclosed in, for example, Japanese Unexamined Patent Publications (Kokai) Nos. 50-105267, 54-152858, 55-136439, 59-16246, and 62-69435.
Japanese Unexamined Patent Publication No. 56-35343 has disclosed a method for removing foreign matters, which have adhered to a shadow mask, using an electron beam generated by a cathode-ray tube itself after the evacuation lock process is complete. According to this method, a deflection circuit in a cathode-ray tube is switched from a raster deflection circuit to a dc deflection circuit in order to produce a spot-like electron beam. The electron beam is aligned with a foreign matter, and a specified beam current is irradiated to the foreign matter continuously for a specified period of time in order to remove the foreign matter. The use of an electron beam generated by a TV cathode-ray tube enables dissolution and removal of an average-size foreign matter for about three minutes.
Among the aforesaid conventional methods for removing foreign matters, a method of applying mechanical vibrations is effective for foreign matters whose adhesions are very weak. In practice, many foreign matters are, however, adhering to a shadow mask persistently or in a fused state, and cannot be removed merely by applying slight vibrations to the shadow mask. Strong vibrations may deform the shadow mask.
In a method of using an electron beam generated by a cathode-ray tube itself, a foreign matter is fused and removed by irradiating a specified quantity of beam current to the foreign matter continuously for a specified period of time. While an electron beam is being irradiated, therefore, the portion of a mask in the vicinity of a foreign matter is also irradiated continuously for the specified period of time.
In a display monitor tube, for example, the diameter of a hole on a shadow mask is about 120 micrometers, and the hole pitch is about 300 micrometers. The size of a foreign matter usually ranges from 100 to 200 micrometers. The diameter of a convergent beam ranges from 500 to 600 micrometers on a tube surface. Assuming that the density distribution of beam current on a mask is a Gaussian distribution and that a 30 value represents a radius of a beam (250 to 300 .mu.m) on the tube surface, energy applied to a foreign matter is at most 10% of all energy. This means that in an actual cathode-ray tube, almost all the beam irradiation energy passes a mask and holes on the mask and enters phosphors.
In the foregoing continuous irradiation method, therefore, a majority of a electron beam passes through the holes on a mask and irradiates not only the mask but also a fluorescent screen at an area in which no space is present between a mask hole and foreign matter or no foreign matter is present. Consequently, the phosphors themselves are heated for a specified period of time.
Continuous heating of a shadow mask resulting from irradiation of beam current during foreign matter removal brings about a temperature rise accompanied by the local thermal deformation of the mask. This causes a relative distance between an electron gun and the mask to change, which results in color misregistration on a screen. Continuous irradiation of a phosphor with an excess quantity of beam current brings about the local thermal damage to the phosphor, which deteriorates the brightness of the phosphor. This results in a defect on a screen.
In an experiment the present inventor conducted using a cathode-ray tube actually, the diameter of an electron beam was reduced to about 700 .mu.m on a tube surface and the electron beam was irradiated for 10 sec with varying beam currents. As a result, it was found that when the quantity of beam current is 40 .mu.A or less, thermally adverse effects are not placed on a mask, but that when the quantity exceeds 50 .mu.A, the mask deforms to cause adverse effects such as color misregistration.
As described above, in a conventional method for removing foreign matters by irradiating an electron beam continuously for a specified period of time, a quantity of beam current per unit area or unit time must be restricted so as not to cause thermal deformation of a mask or thermal damage to phosphors. From a viewpoint of a material property, foreign matters having low fusion points and low sublimation points are removed effectively. Foreign matters having high fusion points and high sublimation points are, however, removed quite ineffectively.
An experimental attempt was made to remove foreign matters using an actual cathode-ray tube under the parameters for continuous beam irradiation that will not cause the aforesaid thermal deformation of a mask. Thereafter, a tube was disassembled and foreign matters which had hardly been removed were analyzed using an electron microbeam analysis (EMPA). As a result, it was confirmed that the materials which had hardly been removed were high-fusion point materials; such as, a graphite conducting material with which the inside of a funnel is coated to hold an anode button and a mask in mutual conduction, glasses, and irons.